Porous pipe acoustic sensor

ABSTRACT

This invention is a dynamic pressure sensor of the porous pipe variety for use in sensing the presence of an acoustic energy source in a moving fluid environment. The sensor is especially adaptable to moving vehicle applications wherein the sensor is located on the moving vehicle and boundary layer interference due to movement of the vehicle and the sensor in air are prime considerations.

United States Patent n 1 Horwath POROUS PIPE ACOUSTIC SENSOR lnventor:Tibor G. Horwath, Alexandria, Va;

Assigneez The UnitedStates of America as represented by the Secretary ofthe Army Filed: Oct. 23, 1970 Appl. No.: 83,560

US. Cl ..340/15, 181/05 R, 18l/0.5 VM, 340/16 R Int. Cl ..1-104b 11/00Field of Search ..181/0.5 R, 0.5 VM; 340/8 R, 340/16 R, 15, 3 T, 7 R

SIGNAL 51 Feb. 6, 1973 [56] References Cited UNITED STATES PATENTS3,485,318 12/1969 Eichler ..181/0.5 R

Primary Examiner-Richard A. Farley Attorney-Harry M. Saragovitz, EdwardJ. Kelly, Herbert Berl and Glen Ovrevik [5 7] ABSTRACT This invention isa dynamic pressure sensor of the porous pipe variety for use in sensingthe presence of an acoustic energy source in a moving fluid environment.The sensor is especially adaptable to moving vehicle applicationswherein the sensor is located on the movingvehicle and boundary layerinterference due to movement of the vehicle and the sensor in air areprime considerations.

5 Claims, 2 Drawing Figures N INTERFERENCE PATENTEDFEH sum 3.715.714

SHEETZUF 2 I 33 PHASE DIFFERENCE QEE SIGNAL 5| I: J \l v FREQUENCY ,52

V00 DISCRIMINATOR DIFFERENCE 34 Y SIGNAL 3 36 3? IM'EYFOR l/ TIBOR s.HORWATH AGENT ATroRxEYs POROUS PIPE ACOUSTIC SENSOR The inventiondescribed herein may be manufactured and used by or for The Governmentof the United States of America for governmental purposes without thepayment to me of any royalties thereon.

BACKGROUND OF THE INVENTION A wide variety of detectors have beenemployed in homing systems for seeking and acquisition of a movingtarget. In general, these homing systems employ detectors sensitive toone or more of the various types of signatures emitted by the movingtarget. It is well known, for example, that temperature sensitivesensors have been employed to track the path of submarines operatingunder water long after their departure from the area. Likewise, infrareddetectors have been employed as homing means to guide-missiles to theexhaust of jet aircraft in flight.

It will be appreciated that all powered moving targets emit an acousticsignature due to motion of the target as well as power plant noise.Heretofore, acoustic signature sensing systems have been largelyrestricted to fixed position detectors in quiet locations. It will beappreciated that homing systems necessitate installation of the sensingmeans on a moving vehicle and this in turn introduces boundary layerinterference noise due to movement of the vehicle and the sensor in air.Such interference noise must be minimized, of course, if acousticsignature detectors are to be utilized effectively. Generally, flownoise reduction techniques, ap-

plicableto acoustic detectors, substantially reduce the signal to noiseratio of the detector. No prior art acoustic detector which affords theessential noise reduction and is sufficiently sensitive for use insophisticated acoustic homing system is presently known.

SUMMARY OF THE INVENTION Acoustic sensors located on a moving vehiclereceive the sound radiated by a target and in the homing systemapplication the target input signal information received is processed toguide the vehicle toward the target. The motion of the sensor relativeto air creates, on the surface of the sensor, a boundary layer ofpotentially unstable flow which causes pressure fluctuations to producea second input signal. Likewise, the surfaces of the vehicle moving inair experience boundary layer pressure fluctuations which radiate towardthe acoustic sensor to produce a third input signal.

It will be appreciated that the basic objective in the design of anacoustic sensor is to enhance the signal to noise ratio of dynamicpressures, that is, to minimize the interference effect of the secondand third input signals on the target signal.

The sensor of the present invention embodies a relatively largesensitive surface area which enables a direct summation of localboundary layer pressures and thus affords a more restricted acceptanceregion in spectral space as compared to that obtained with a group ofsmall sensors, each effectively measuring the local pressure at a set ofpoints, with electronic means for consideration of the electricaloutputs of the individual sensors. 7 i

The large sensitive area of the sensor of the present invention consistsof a porous cylindrical surface. The sensor, which may betermed a porouspipe sensor, is designed to receive the three principal input signalsthrough its porous wall. A uniform frequency response is achieved whenacoustic waves inside the porous pipe propagating toward the front ofthe pipe are absorbed. Optimum performance, maximum sensitivity andsignal to noise ratio is obtained when the reflection coefficient of thewaves propagating toward the microphone end of the pipe is unityindependent of frequency.

The uniform frequency response and the maximum sensitivity of the sensorare achieved by the introduction of an internally disposed reflectivesurface of selected configuration.

' The disclosed novel sensor structure effectively eliminates the noiseinterference of the second and third input signals on the target signal.In addition, this structural configuration provides an acoustic sensorwherein magnitude of the sensed signal is indicative of the angle ofincidence of the signal.

These features and other significant objects of the invention willbecome apparant from a clear understanding of the invention for whichreference is had to the description of a preferred embodiment of theinvention and the drawings wherein:

FIG. 1 is cross section view of the porous pipe sensor of this inventionshowing the internal structure thereof in a preferned embodiment,together with its associated microphone.

FIG. 2 pictorally depicts an array of porous pipe sensors in a typicalorientation with a typical signal processor connected to the sensors forobtaining guidance information.

DESCRIPTION OF THE PREFERRED EMBODIMENTS tubular member 11 and extendssubstantially the entire length of the tubular member II. As indicatedat 16, the tubular member 11 is threaded at the nose cone end thereoffor simplified nose cone and reflective surface assembly purposes.

In accordance with the preferred embodiment of the invention, the thinwall of the tubular member 11 is made of sintered stainless steelparticles of selected size and the wall is identified by itsmanufacturer, Mott Metallurgical Corporation, Farmington, Connecticut,06032, in accordance with micron size of the sintered stainless steelparticles. This particular thin wall product is more commonly used inair filtration applications and has been found to have a substantiallyuniform porosity as measured by its flow resistance characteristic.

where p pressure drop across the sample (Newtons/m) u volume velocity ofairflow across the sample (m /sec) 1- flow resistance (MKS acousticohms) For purposes of this invention, flow resistance F, is measured atlow values of pressure drops Ap across the porous wall, i.e., in theregion Ap= 0.1 inch 1.0 inch water.

it has been found that for acoustic waves at low intensities, the flowresistance F I is constant independent of amplitude. in a typicalsensor, the porous tubular member 11 could be one-sixteenth inch inthickness, l2 inches in length, one-half inch in diameter, and made of 2micron material having a flow resistance of approximately 105 pc,measured at low values of pressure differential across the porous wallwhere p is the density of the fluid medium and c is the velocity ofsound in the fluid medium. Where air is the fluid medip (density of thefluid medium) 1,293 kgm c (velocity ofsound in thefluid medium) 3,3410 mThe sintered stainless steel particles material disclosed as thesensitive side wall surface herein may be physically described as arelatively thin material made of metallic particles of micron size andhaving a multitude of randomly distributed and substantially closelyspaced, finite, nondescript channels which determine the flow resistancecharacteristic of the material in selected fluid mediums.

While the porous pipe tubular member 11 is shown as cylindrical in theillustrated embodiment with a wall of uniform thickness, it will beappreciated that other elongated configurations of oval, triangular,orrectangular cross section, for example, may be utilized in selectedapplications with the outer configuration dependent primarily uponaerodynamic considerations and the inner configuration largely dependentupon standing wave considerations.

Furthermore, it will be appreciated that it is within the purview ofthis disclosure to substitute other porous wall materials having asuitable flow resistance characteristic and to adapt same, if necessary,to support wave energy within the tubular body 11. However, it should beunderstood that nonporous wall surfaces which have been perforated toprovide a selected flow resistance are not interchangeable with porouswall surfaces in this invention. in particular, it has been found thatfor a given size sensor, the maximum flow impedance obtainable, whichwould be otherwise suitable for use, is several orders of magnitude toolow for use in this invention. Moreover, perforations afford anirregular surface which creates undesireable turbulence at the sensitivesurface and have been found to generate a side lobe interference factor,which dependent upon size of the perforations and spacing therebetween,may create a noise source of considerable consequence.

As shown in FIG. 1, the microphone 13 may be connected to the porouspipe tubular member 11 by a nonporous, loss free acoustical conduit 14having a cross section configuration substantially the same as theporous pipe tubular member 11. Both condenser microphones (Bruel &Kjaer, type 4133) and piezoelectric microphones Lafayette No. 99-4509)have been found suitable for use in this invention. in-

deed, the selection of the type of microphone is not criticaland manydifferent varieties may be employed. in particular, it has been foundthat the type of microphone selected has little, if any effect, on thedirectivity characteristic of the sensor, which is, of course, ofparticular concern in homing system applications.

it will be appreciated that it is within the purview of this disclosureto incorporate various propagation mode transition devices, not shown,in lieu of the wave energy transmission means shown in the drawing 14 ifthe selection of a particular type of microphone necessitates apropagation mode transition.

In consideration of the configuration of the preferred embodiment, itwill be recognized that the degree of flow noise rejection reflected inthe signal to noise ratio of an acoustic signal received by the porouspipe sensor depends to a large extent on the standing wave ratio insidethe porous pipe member 11 due to the reflection coefficient of the frontend termination. The signal to noise ratio is optimum if the front endof the member 1 l is appropriately terminated to be nonreflective.

An effectively nonreflective termination is achieved in this inventionby internal disposition of the reflective surface 15 which extends fromone end of the elongated member 11 to the other.

It has been found that in the relatively low frequency application ofthe sensor of this invention the flow impedance characteristic F of theporous material and the internal dimensions (length L and diameter D)should be in the following relation:

where p =is the density of the fluid medium c =is the velocity of soundin the medium Likewise, it has been found that in the uniform porosity,cylindrical tube configuration shown in FIG. 1, the shape of theinternal cone should afford a taper from the axis, beginning at themicrophone end of the elongated member 11 and expanding outward. it hasbeen determined that a parabolic curvature of the taper, as shown in thedrawing, affords the best results in the case of a uniformly reflectivesurface. However, it will be recognized that the configuration of thereflective surface 15 may vary in accordance with other structuralvariations. For example, in the event the flow resistance of the porouspipe member 1 l was graduated along its length with greater flowresistance at the front end, the conical configuration of the reflectivesurface might be broadened in the region of its base.

FIG. 2 shows an array of four porous pipe sensors 21, 22, 23 and 24 in atypical homing system orientation adapted for mounting on a vehicle, notshown, and moveable in the three coordinates x, y and z.

The sensors 21, 22, 23 and 24 are electrically connected to respectivenarrow band pass channels 31, 32, 33 and 34, each comprising a cascadeconnection of preamplifier modulator, and band pass filter as shown. Thechannels 31 and 33 are connected to a phase measuring circuit 41 toprovide a first phase differential signal and the channels 32' and 34are connected to a phase measurement circuit 42 to provide a secondphase differential signal.

Tracking a narrow band filtering of a selected harmonic tone of thetarget signal is achieved by heterodyne technique. A variable frequencyoscillator 51, which may be voltage controlled oscillator, is connectedto the modulator in each of the channels 31, 32, 33 and 34 to modulatethe sensor output.

In a typical case, a symmetrical voltage controlled oscillator, onewhich suppresses the carrier frequency of the oscillator, f at itsoutput, is employed as the modulator. Identical fixed bandwidth bandpass filters are employed in each of the channels 31, 32, 33 and 34 andeach band pass filter is tuned about a center frequency f., to capture apart of the upper sideband of the modulated signal entering the bandpass filter. Thus, a change in frequency,f,,, of the modulator effec-.tively enables a scanning of the frequency spectrum of the sensor signalwithin bandwidth limits of the band pass filters.

In the homing system application, the tracking function is achieved bythe addition of a frequency discriminator 52 responsive to the outputsof each of the channels 31, 32, 33 and 34. The output of the frequencydiscriminator 52 controls the frequency, f,, of the modulator 51. Thus,if a significant portion of the modulated signal falls outside the passband of the filters, the output of the channels is reduced in magnitudeand the frequency discriminator responds in a conventional feedbackmanner to shift the modulator frequency, f such that the modulatedtarget signal falls within the pass band of the filters.

The phase measuring circuits 41 and 42 each function in a relativelyconventional manner, first clipping each input signal, then obtainingpulses from the axis crossings of the clipped wave train and thereaftergenerating a rectangular wave train whose ON duration is proportional tothe time difference between axis crossings of the two input signals.Thus, the mean valve output of each phase measuring circuit is a linearfunction of the phase delay between pairs of sensors.

While the electronic circuitry of the various elements of the channels31, 32, 33 and 34 as well as associated modulator, frequencydiscriminator, and phase measuring means may be conventional, it hasbeen found that phase matching throughout is vital to operation of thesystem. That is, the phases of each pair of sensors and associatedchannels must be well matched throughout.

In practice, thisphase matched requirement means that in the normalfrequency range of the acoustic sensor, 300 1,000 cycle/sec, (l) theporous pipes 11 should have nearly identical physical dimensions andporosity characteristics, as well, including internal reflectivestructure (2) the microphones in each pair of channels should be phasematched with respect each other and their respective porous pipesections (3) the preamplifiers should be phased matched and (4) thenarrow band filters in each channel should be phase matched.

Other elements of the electronic circuitry, such as the amplitudeclippers and the modulator 5! usually have a rather wide bandwidth andconsequently do not present a phase matching problem.

It will be seen that the dynamic performance of the homing. systemillustrated in FIG. 2 is controlled primarily by the responsecharacteristic of the band pass filters in each of the channels 31, 32,33 and 34. Accordingly, it has been found desireable to make theresponse of the feedback loop within the helerodyne oscillator circuitryfaster than the response of the filters. This provides a steady stategain with gain dependent upon the signal to noise ratio (SNR) of thesensor output.

In determining the signal to noise ratio, the flow noise signal derivedfrom movement of the sensor vehicle, is considered as the main source ofnoise, at each sensor and the noise is assumed to have the samemagnitude, frequency spectrum and bandwidth at each sensor. For a targetsignal of selected value of db relative to 2 X l0- microbar rumspressure at the location of the sensor travelling with a velocity of 200feet/sec a signal to noise ratio of 14 db is readily obtainable, if thesignal and the flow noise are measured at a frequency of 1,000cycles/sec in a 28 percent relative bandwidth.

in comparison, the signal to noise ratio obtained utilizing the sametarget signal in an identical noise environment when porous pipe sensorsof identical size and configuration except for the absence of theinternal reflective surface indicated at 15 in FIG. 1 has been measuredas 6 8 db.

It will be appreciated that the acoustic sensor exemplarily depictedherein may be modified in accordance with standard aerodynamic practiceas required in selected applications without departure from the purviewof this disclosure.

I claim 1. An acoustic sensor for use in sensingthe presence of anacoustic energy source in a moving fluid medium environment comprisingan elongated porous pipe section of selected porosity, length and crosssection configuration, said porous pipe section having a relatively thinside wall with a multitude of randomly distributed and substantiallyclosely spaced, finite, nondescript channels determining a substantiallyconstant flow resistance, F,, in said fluid medium at relatively lowlevel pressure differential thereacross; a-pressure sensitive energytransition means having an electrical signal output proportional to apressure wave input; means connecting said energy transition means to afirst end of said elongated porous pipe section; a reflective conicalsurface internally disposed in said porous pipe section and extendingsubstantially the length thereof with the base of said conical surfaceconnected to the second end of said porous pipe section.

2. An acoustic sensor as defined in claim 1 wherein said porous pipesection is cylindrical and the taper of said conical reflective surfaceis parabolic.

3. An acoustic sensor as defined in claim 2 wherein said constant flowresistance, F and the length L and diameter D of said cylindrical porouspipe section are in the relation where p is the density of the fluidmedium and c is the velocity of sound in the medium.

4. An acoustic sensor as defined in claim 3 wherein said thin side wallis made of sintered stainless steel particles of selected micron sizeand said thin side wall has a substantially uniform porosity oversubstantially the entire surface area thereof.

5. An acoustic sensor as defined in claim 2 wherein a nose cone isconnected to said second end of said porous pipe section, said nose coneadapted to minimize pressure wave turbulence along the outer surface ofsaid porous pipe section in said moving fluid medium environment.

1. An acoustic sensor for use in sensing the presence of an acousticenergy source in a moving fluid medium environment comprising anelongated porous pipe section of selected porosity, length and crosssection configuration, said porous pipe section having a relatively thinside wall with a multitude of randomly distributed and substantiallyclosely spaced, finite, nondescript channels determining a substantiallyconstant flow resistance, Fx, in said fluid medium at relatively lowlevel pressure differential thereacross; a pressure sensitive energytransition means having an electrical signal output proportional to apressure wave input; means connecting said energy transition means to afirst end of said elongated porous pipe section; a reflective conicalsurface internally disposed in said porous pipe section and extendingsubstantially the length thereof with the base of said conical surfaceconnected to the second end of said porous pipe section.
 1. An acousticsensor for use in sensing the presence of an acoustic energy source in amoving fluid medium environment comprising an elongated porous pipesection of selected porosity, length and cross section configuration,said porous pipe section having a relatively thin side wall with amultitude of randomly distributed and substantially closely spaced,finite, nondescript channels determining a substantially constant flowresistance, Fx, in said fluid medium at relatively low level pressuredifferential thereacross; a pressure sensitive energy transition meanshaving an electrical signal output proportional to a pressure waveinput; means connecting said energy transition means to a first end ofsaid elongated porous pipe section; a reflective conical surfaceinternally disposed in said porous pipe section and extendingsubstantially the length thereof with the base of said conical surfaceconnected to the second end of said porous pipe section.
 2. An acousticsensor as defined in claim 1 wherein said porous pipe section iscylindrical and the taper of said conical reflective surface isparabolic.
 3. An acoustic sensor as defined in claim 2 wherein saidconstant flow resistance, Fx, and the length L and diameter D of saidcylindrical porous pipe sEction are in the relation where Rho is thedensity of the fluid medium and c is the velocity of sound in themedium.
 4. An acoustic sensor as defined in claim 3 wherein said thinside wall is made of sintered stainless steel particles of selectedmicron size and said thin side wall has a substantially uniform porosityover substantially the entire surface area thereof.